با سلام
من برای دانلود فایل pdf از این کد استفاده کردم حالا  میخوام براساس شرط دانلود انجام بشه خطا میده؟

public partial class barname_azmon : System.Web.UI.Page

{
    protected void Page_Load(object sender, EventArgs e)
    {
    }
          private DataTable GetData(SqlCommand cmd)
    {
        DataTable dt = new DataTable();
        //String strConnString = System.Configuration.ConfigurationManager.ConnectionStrings["9dey"].ConnectionString;
        SqlConnection cnn = new SqlConnection(SavePlace.CString);
        SqlDataAdapter sda = new SqlDataAdapter();
        cmd.CommandType = CommandType.Text;
        cmd.Connection = cnn;
        try
        {
            cnn.Open();
            sda.SelectCommand = cmd;
            sda.Fill(dt);
            return dt;
        }
        catch 
        {
            return null;
        }
        finally
        {
            cnn.Close();
            sda.Dispose();
            cnn.Dispose();
        }
    }

    private void download (DataTable dt)
    {
        Byte[] bytes = (Byte[])dt.Rows[0]["data"];
        Response.Buffer = true;
        Response.Charset = "";
        Response.Cache.SetCacheability(HttpCacheability.NoCache);
        Response.ContentType = dt.Rows[0]["ContentType"].ToString();
        Response.AddHeader("content-disposition", "attachment;filename="
        + dt.Rows[0]["Name"].ToString());
       
        Response.BinaryWrite(bytes);
        Response.Flush(); 
        Response.End();
    }


    protected void Retreive_Pdf(object sender, EventArgs e)
    {
        string strQuery = "select * from namaish_pdf where mozoe='برنامه امتحانی دوم علوم تجربی'";
        SqlCommand cmd = new SqlCommand(strQuery);
        //cmd.Parameters.Add("@mozoe", SqlDbType.NVarChar).Value = '1' ;
        DataTable dt = GetData(cmd);
        if (dt != null)
        {
            download(dt);
        }
    }
}

از این خط خطا میگیره

Line 44:         Byte[] bytes = (Byte[])dt.Rows[0]["data"];